Optical pickup apparatus and optical disc apparatus using same

ABSTRACT

An optical pickup apparatus includes a semiconductor laser for emitting a laser light, an objective lens for irradiating the light flux emitted from the semiconductor laser onto an optical disc, and a light detector for receiving the light flux reflected from the optical disc. The light detector has a light receiving part that comprises five regions of a region  1 , a region  2 , a region  3  and a region  4.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.11/680,705, filed Mar. 1, 2007, the contents of which are incorporatedherein by reference. This application relates to U.S. application Ser.Nos. ______, ______, and ______, filed Oct. 30, 2007, which aredivisional applications of U.S. Ser. No. 11/680,705, filed Mar. 1, 2007.

INCORPORATION BY REFERENCE

The present application claims priorities from Japanese applicationsJP2006-283248 filed on Oct. 18, 2006, JP2006-283245 filed on Oct. 18,2006, the contents of which are hereby incorporated by reference intothis application.

BACKGROUND OF THE INVENTION

The present invention relates to an optical disc apparatus for recordinginformation on or reproducing information from an optical disc, and toan optical pickup apparatus used for the same. More particularly, thepresent invention relates to an optical disc apparatus for recordinginformation on or reproducing information from an optical disc that hasa plurality of laminated information recording layers, and to an opticalpickup apparatus used for the same.

A technology of a multilayer optical disc having laminated informationrecording layers has been studied as a method of increasing the storagecapacity of the optical disc. In the standard of a DVD (DigitalVersatile Disc), BD (Blue-ray Disc) and HD-DVD (High Density DigitalVersatile Disc), a two-layer optical disc is commercialized in which twoinformation recording layers are laminated at an interval of about 20 to55 μm. In addition, a three- or more-layer optical disc has also beenstudied as a technology of achieving a larger capacity.

When recording information on or reproducing information from themultilayer optical disc, it is necessary to eliminate as much aspossible the offset of a servo signal such as a focus error signal or atracking error signal caused by a stray light from other layer.

A method of eliminating the effect due to the stray light from otherlayer is described, for example, in an article of “Journal of Instituteof Electronics Information and Communication Engineers” CPM2005-149(2005-10), which describes about the placement of a trackingphotodetector in a region free of a stray light from other layer.

However, the above article (CPM2005-149 (2005-10)) does not describeabout the effect that the stray light has on the focus error signal.

Furthermore, in the above article (CPM2005-149 (2005-10)), it isrequired to dispose a light receiving part for the tracking error signaloutside the stray light from other layer that occurs around the lightreceiving part for the focus error signal, thus the size of a lightdetector being increased.

A background art of the optical pickup apparatus is proposed in aJapanese Laid-open Patent Application JP-A-9-223321. In the Laid-openPatent Application JP-A-9-223321, PROBLEM TO BE SOLVED reads as follows:To provide an optical information reproducing apparatus that can besimplified by reducing the number of optical parts, and to provide amethod of adjusting the optical information reproducing apparatus thatenables the adjustment of a tracking error signal in accordance with thecharacteristic of an optical disc. SOLUTION reads as follows: Theoptical information reproducing apparatus comprises: an optical pickuphaving an objective lens for irradiating the optical disc with light; afirst dividing means for dividing a light spot of a light emitted fromthe optical disc substantially perpendicularly to the directionequivalent to a track to form a light spot on an end region and a lightspot on a middle region relative to the center of the light spot; asecond dividing means for further dividing the light spots on the endregion and middle region in substantially parallel to the directionequivalent to the track of the optical disc; a light receiving elementhaving a plurality of light receiving cells for receiving the lightdivided by the first and second dividing means; a light spotdisplacement signal detecting means for computing the outputs of thelight receiving cells that receive the light on the middle regiondivided by the second dividing means to detect the relative displacementof the light spots on the light receiving element; a tracking errorgenerating means for computing the outputs of the light receiving cellsthat receive the light on the middle region divided by the seconddividing means to detect a relative displacement between the track andobjective lens; an offset correction means for correcting the offset ofthe tracking error signal by the computing the output signal of thelight spot displacement signal detecting means and the output signal ofthe tracking error generating means; an objective lens driving devicefor driving the objective lens in the direction across the track of theoptical disc; a tracking control means for drive-controlling theobjective lens driving device; and a switching means for switching theinput of the tracking control means to the output of the light spotdisplacement signal detecting means during an access, and for switchingthe input of the tracking control means to the output of the trackingerror generating means via the offset correction means duringreproducing of the information of the optical disc.

SUMMARY OF THE INVENTION

In the optical pickup apparatus, generally, in order to correctlyirradiate a spot on a given record track in the optical disc, anobjective lens is displaced in the focusing direction through thedetection of a focus error signal, thus the objective lens beingadjusted in the focus direction. Furthermore, the objective lens isdisplaced in the radial direction of a disc shape recording mediumthrough the detection of a tracking error signal, thus the trackingadjustment is performed. These signals allow the objective lens to beposition-controlled.

While a push pull method is known as a tracking error signal detectionmethod of the above error signal detections, it has a problem that adirect current fluctuation (referred to as a DC offset hereinafter) isprone to occur. Therefore, a differential push pull method is widelyused that is capable of reducing the DC offset.

The differential push pull method divides a light flux into a main lightflux and a sub light flux through a diffraction grating and reduces theDC offset using a spot of the main light flux and a spot of the sublight flux in the radial direction.

However, since the differential push pull method forms a plurality ofspots on the optical disc, light use efficiency of the main light fluxdecreases. The main light flux not only generates a focus error signaland a tracking error signal, but also has a function of forming a recordmark on the recording optical disc. When performing recording on therecording optical disc, its writing speed becomes faster the larger thelight amount of the main light flux on the disc is. Therefore, it isdisadvantageous to use the diffraction grating for an outward opticalsystem from a viewpoint of writing speed.

Therefore, in the above JP-A-9-223321, one spot is formed on the disc,and its reflective light is divided into a plurality of regions, thusinspecting a stable tracking error signal free of the DC offset even ifthe objective lens is displaced in the tracking direction. Thisstructure has an advantage that the writing speed can be increasedwithout reducing the light use efficiency. (referred to as a one-beammethod hereinafter)

However, when the detector is divided into regions as in the aboveJP-A-9-223321, a problem occurs in a recording type optical disc, suchas, for example, BD-RE or BD-R. In the recording type optical disc,there exist a region where recording is not performed (referred to as anunrecorded region hereinafter) and a region where recording is alreadyperformed (referred to as a recorded region hereinafter). When theregion is divided as described in the above JP-A-9-223321, it isimpossible to reduce the offset of the tracking error signal occurringat the boundary between the unrecorded region and recorded region on thedisc, posing a problem.

It is an object of the present invention to provide an optical pickupapparatus capable of obtaining an stable servo signal and an opticaldisc apparatus equipped with the same.

In order to solve the above problems, the optical pickup apparatusaccording to the present invention comprises: a light source; anobjective lens for focusing a light flux emitted from the light sourceon the optical disc; a dividing element for dividing the light fluxreflected from the optical disc into a plurality of light fluxes; acondenser lens for condensing the light flux reflected from the opticaldisc; and a light detector for receiving the light flux condensed by thecondenser lens with a plurality of light receiving parts to convert itinto an electrical signal. The dividing element has a first dividedregion disposed almost on the center; a second divided region comprisedof four regions which are divided by a first dividing line and disposedalong the direction of the first dividing line to sandwich the firstdivided region; and a third divided region comprised of four regionswhich are divided by a second dividing line perpendicular to the firstdividing line and disposed along the direction of the second dividingline to sandwich the first divided region. Each of the first to thirddivided regions is structured such that when a target informationrecording layer of the optical disc is brought into focus, a reflectivelight flux from the target information recording layer is focused on thelight receiving parts of the light detector, and a reflective light fluxfrom a recording reproducing layer other than the target informationrecording layer is not irradiated onto the light receiving parts of thelight detector.

The present invention enables the one beam tracking method to obtain astable focus error signal and tracking error signal.

Furthermore, the present invention improves the offset of the trackingerror signal occurring at the boundary between the unrecorded region andrecorded region, which is a problem of the above one-beam method. Morespecifically, the present invention provides an optical pickupapparatus, an optical information reproducing apparatus or an opticalinformation recording and reproducing apparatus that uses a noveltracing error detecting means capable of detecting a stable trackingerror signal even if there exists a boundary between the unrecordedregion and recorded region on the optical disc.

It is an object of the present invention to provide the optical pickupapparatus and optical information recording and reproducing apparatusthat are capable of detecting a stable tracking error signal.

The above objects are implemented by the structure described in theclaim as an example.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an optical pickup apparatus as an embodimentof the present invention;

FIG. 2 is a diagram showing the shape of a polarizing diffractiongrating in an embodiment of the present invention;

FIG. 3 is a diagram showing a light detector and an light pattern in anembodiment of the present invention;

FIGS. 4A to 4F are diagrams showing how light patterns in an embodimentof the present invention change;

FIGS. 5A to 5E are diagrams showing how light patterns in an embodimentof the present invention change;

FIGS. 6A to 6E are diagrams showing the shapes of light patterns on atwo-layer disc in an embodiment of the present invention;

FIG. 7 is a diagram showing an embodiment 2 of the polarizingdiffraction grating in the present invention;

FIG. 8 is a diagram showing the shape of light patterns of theembodiment 2 of the polarizing diffraction grating in the presentinvention;

FIG. 9 is a diagram showing an embodiment 3 of the polarizingdiffraction grating in the present invention;

FIG. 10 is a diagram showing the shape of light patterns of theembodiment 3 of the polarizing diffraction grating in the presentinvention;

FIG. 11 is a schematic diagram of the optical disc apparatus equippedwith the optical pickup apparatus according to the present invention;

FIG. 12 is a diagram explaining the disposition of an optical pickupapparatus and an optical disc apparatus in an embodiment 4;

FIG. 13 is a diagram explaining an optical pickup apparatus using a onebeam method in the embodiment 4;

FIG. 14 is a diagram explaining a light receiving part or a polarizingdiffraction grating of the embodiment 4 in the present invention;

FIG. 15 is a diagram explaining the DC offset when the objective lens ofFIG. 12 is displaced on the inner and outer periphery;

FIGS. 16A to 16C are diagrams schematically explaining the offsetoccurring at the boundary between an recorded region and a recordedregion in the one beam method of the embodiment 4;

FIGS. 17A and 17B are diagrams comparing the characteristic of theunrecorded region and recorded region in the embodiment 4 withJP-A-9-223321;

FIGS. 18A and 18B are diagrams explaining the effect of the presentinvention by means of the light receiving part or a difference in themethod of dividing the surface of the diffraction grating in theembodiment 4;

FIGS. 19A and 19B are diagrams showing a light receiving part or adiffraction grating surface other than that shown in FIG. 14 in theembodiment 4;

FIG. 20 is a diagram explaining an optical pickup apparatus using theone beam method in the embodiment 4;

FIG. 21 is a diagram explaining a light receiving part or diffractiongrating surface in an embodiment 5;

FIGS. 22A and 22B are diagrams comparing the characteristic of theunrecorded region and recorded region in the embodiment 5 withJP-A-9-223321;

FIGS. 23A and 23B are diagrams showing a light receiving part or adiffraction grating surface other than that shown in FIGS. 19A and 19Bin the embodiment 5;

FIG. 24 is a diagram explaining an optical pickup apparatus using theone beam method in an embodiment 6;

FIGS. 25A and 25B are diagrams explaining a light receiving part in theembodiment 6;

FIG. 26 is a diagram explaining an optical pickup apparatus using theone beam method other than FIG. 24 in the embodiment 6;

FIG. 27 is a diagram explaining an optical pickup apparatus using theone beam method in an embodiment 7;

FIG. 28 is a diagram explaining a diffraction grating surface in theembodiment 7;

FIG. 29 is a diagram explaining light receiving parts in the embodiment7;

FIG. 30 is a diagram explaining an optical reproducing apparatus in anembodiment 8; and

FIG. 31 is a diagram explaining an optical recording and reproducingapparatus in an embodiment 9.

DESCRIPTION OF THE EMBODIMENTS

First, embodiments of the optical pickup apparatus according to thepresent invention will be described. The optical pickup apparatusaccording to the present invention is structured such that for example areflected light from a multilayer disc is divided into a plurality ofreflected light fluxes having different outgoing directions and thedivided light fluxes are focused on different poisons on a lightdetector. Furthermore, the optical pickup apparatus according to thepresent invention is structured such that a photo focus error signal isdetected using a reflected light flux passing through a region that doesnot include the light flux center out of the reflected light fluxespassing through a dividing element according to a knife edge method, anda tracking error signal is detected using a reflected light flux passingthrough a region that does not include the light flux center. Moreover,when a target layer is focused, each region of the dividing element andthe light receiving parts are disposed to prevent a stray light fromother layer from entering the light receiving parts for a servo signalof the light detector.

An embodiment of the optical pickup apparatus and optical disc apparatusequipped with the same according to the present invention will bedescribed with reference to FIG. 1 to FIGS. 5A to 5E.

Embodiment 1

FIG. 1 is a diagram showing the structure of the optical pickupapparatus according to the present invention.

A laser light 2 emitted from a semiconductor laser 1 is reflected by apolarizing beam splitter 3 and converted into a parallel light flux by acollimate lens 4. The parallel light flux passes through a polarizingdiffraction grating 5 and a one-quarter wave plate 6, and is focused bya objective lens 7 on an optical disc 8. The optical disc 8 is providedwith a recording and reproducing layer (information recording layer)comprising two layers of a first layer 9 and a second layer 10, witheach layer being formed with a track (not shown) in the direction ofarrow 11.

When any of the two recording and reproducing layers of the optical discis in focus, the laser light is reflected by the optical disc 8 andpasses through the objective lens 7 and one-quarter wave plate 6. Then,the laser flux is divided by the polarizing diffraction grating 5 toenter a plurality of regions, with each light flux advancing indifferent directions. Then, the light flux passes through the collimatelens 4 and polarizing beam splitter 3, and is focused on a lightdetector 12.

A plurality of light receiving parts 13 are formed on the light detector12, and the light flux divided by the polarizing diffraction grating 5is irradiated onto each of the light receiving parts 13. Electricalsignals are outputted from the light detector 12 in response to theamount of light irradiated onto the light receiving parts 13. Theoutputs are computed to generate a focus error signal and a trackingerror signal.

In the description hereinafter, when the optical pickup apparatus isdisposed to face the optical disc for the purpose of recording orreproducing, the direction perpendicular to the surface of the opticaldisc 8 is defined as a Z axis, the track direction as a Y axis, and thedirection perpendicular to the track as an X axis. The Z axis issubstantially parallel with the optical axis of the light flux emittedfrom the objective lens 7.

FIG. 2 shows the shape of the polarizing diffraction grating 5 ofFIG. 1. The polarizing diffraction grating 5 is divided into a pluralityof regions. In FIG. 2, solid lines show boundary lines, while analternate long and two short dashes line schematically shows the outsideshape of the light flux of a laser light, and shadow areas schematicallyshow a push pull pattern occurring due to the track of the optical disc.

The polarizing diffraction grating 5 is formed with a dividing line 15in the Y-axis direction that passes through the light flux center 14,and with a dividing line 16 in the X-axis direction. The polarizingdiffraction grating 5 is also formed with a divided region (firstdivided region) that comprises four regions (regions C1 to C4) which arepoint-symmetrical to each other with respect to the light flux center 14and includes the light flux center 14; a divided region (second dividedregion) that comprises four regions (regions A1 to A4) which arepoint-symmetrical to each other with respect to the light flux center14, does not include the light flux center 14, and includes part of thedividing line 16 in the X-axis direction; and a divided region (thirddivided region) that comprises four regions (regions B1 to B4) which arepoint-symmetrical to each other with respect to the light flux center14, does not include the light flux center 14, and includes part of thedividing line 15 in the Y-axis direction.

When the optical pickup apparatus is disposed to face the surface of theoptical disc during recording or reproducing, the dividing line 15 issubstantially perpendicular to the track direction of the optical disc,and the dividing line 16 is substantially parallel to the trackdirection of the optical disc.

The regions A1 to A4 are divided by the dividing line 16 in the X-axisdirection passing through the light flux center 14, two dividing lines17 in the Y-axis that do not pass through the light flux center 14, fourdividing lines 18 in the X-axis direction that do not pass through thelight flux center 14, and four dividing lines 19 around the light fluxcenter 14 that form angles of 30 degrees with respect to the Y-axisdirection. The interval u in the Y-axis direction of the four dividinglines 18 running in the X-axis direction is set to include a push pullpattern in the range of about 55% to 70% of the light flux diameter inthe embodiment.

The regions A1 to A4 are disposed to sandwich the regions C1 to C4. Theregions A1 to A4 are formed such that the regions A1 and A2 areline-symmetrical to the regions A4 and A3, respectively, with respect tothe dividing line 15.

Regions B1 to B4 are also provided to sandwich the regions C1 to C4. Theregion B1 is provided to be line-symmetrical to the region B2 withrespect to the dividing line 16, while the region B4 is provided to beline-symmetrical to the region B3 with respect to the dividing line 16.

The interval w between the two dividing lines 17 running in the Y-axisdirection is set to be as small as possible under the condition that theregion A includes the push pull pattern and the stray light does notenter the light receiving parts 13 depending on the shape of the lightreceiving parts 13 of the light detector 12. In the embodiment, it isset within the range of about of 25% to 30% of the light flux diameter.The dividing lines 19 that form angles of 30 degrees with respect to theY-axis direction are provided to prevent the entry of the stray lightinto the light receiving parts 13.

The interval v between two dividing lines 20 running in the X-axisdirection on the boundary between the region B and region C is set to beas small as possible under the condition that the stray light does notenter the light receiving parts 13 in response to the shape of the lightreceiving parts 13 of the light detector 12.

The shape of diffraction grating formed on the region C1 is the same asthat formed on the region C3, and that formed on the region C2 is thesame as that formed on region C4. However, the shapes of diffractiongratings formed on other regions are different with each other. In eachdiffraction grating, the light flux is divided into plus(+)/minus(−)first-order diffracted light before being irradiated onto the lightdetector 12.

FIG. 3 shows the shape of the light receiving parts 13 of the lightdetector 12, and the shapes of light patterns irradiated onto the lightdetector. In FIG. 3, the light pattern of only the reflected light fromthe recording and reproducing layer is shown, and the light pattern ofthe stray light from other layer is not shown.

When the recording and reproducing layer is in focus, the laser lightreflected from the recording and reproducing layer is focused at a point21 on the light detector 12, and is irradiated onto the light receivingpart 13 comprising 18 light receiving parts of A to Z formed on thelight detector 12.

Light receiving parts M, N, O and P detect a focus error signal using adouble knife edge method. If the light flux of a plus/minus first-orderdiffracted light which is diffracted on the region A1 and irradiatedonto the light detector 12 is represented as a1+ and a1−, then a lightflux a1− is irradiated onto the boundary between the light receivingparts M and N, a light flux a2− is irradiated onto the boundary betweenthe light receiving parts P and O, a light flux a3− is irradiated ontothe boundary between the light receiving parts P and N, a light flux a4−is irradiated onto the boundary between the light receiving parts M andO. When the outputs from light receiving parts A to J are represented bya to j, respectively, and the outputs of light receiving parts M to Tare represented by m to t, respectively, an focus error signal (FES) isobtained by the following computing equation.

(FES)=(m+p)−(n+o)

Light receiving parts E, F, G and H and light receiving parts Q, R, Sand T are disposed outside light receiving parts A, B, C and D and lightreceiving parts M, N, 0 and P, respectively. The light receiving partsA, B, C and D are irradiated with light fluxes a1+, a2+, a3+ and a4+.The light receiving parts E, F, G and H are irradiated with light fluxesb1+, b2+, b3+ and b4+. The light receiving parts Q, R, S and T areirradiated with light fluxes b1−, b2−, b3− and b4−. They are used fordetecting tracking error signals. The outputs of the light receivingparts Q, R, S and T shall be q, r, s and t, respectively.

The tracking error signal (TES) according to the push pull method isobtained by the following computing equation.

(TES)=((a+e+b+f)−(c+g+d+h)−K((q+r)−(s+t))

where K is a constant, and the value of K is determined such that anoffset does not occur to (TES) when the objective lens 7 moves in theX-axis direction due to a tracking operation.

The tracking error signal (DPD) according to a DPD method is obtained bydetecting the phase difference between (a+e, c+g) and (b+f, d+h).

Light receiving parts I and J are disposed outside the light receivingparts E, F, G and H, and the light receiving part I is irradiated withlight fluxes c1+ and c3+, while the light receiving part J is irradiatedwith light fluxes c2+ and c4+. These are combined with other signals andare used for the detection of reproduced signals (RF), which areobtained using the following computing equation.

(RF)=a+b+c+d+e+f+g+h+i+j

Light fluxes c1−, c2−, c3− and c4− are irradiated onto the place wherethe light receiving parts do not exit, and so these signals are not usedfor the signal detection.

FIGS. 4A to 4F show how the light pattern of each light flux that isirradiated onto the light detector 12 changes during defocusing, and thewaveform of the focus error signal (FES). FIGS. 4A to 4C, 4E and 4Fcorrespond to (a) to (e) of FIG. 4D.

The light pattern of a plus first-order diffracted light 22 is shown bya lattice pattern and the light pattern of a minus first-orderdiffracted light 23 is shown by oblique lines. When the focused focalpoint is positioned at (c), the light pattern is focused on the boundaryof the light receiving parts M to P and at this time the focus errorsignal becomes 0. As the defocusing increases, the light pattern becomeslarger. At (b) or (d), the focus error signal reaches a maximum orminimum value. Moreover, at (a) and (e) where the light pattern becomeseven larger, the light receiving parts 13 cease to be irradiated withlight, with the focus error signal becoming 0.

As the defocusing increases, the light pattern becomes larger around thefocus point (c), and at this time, the light pattern on the regions C1to C4 also becomes larger. However, the light pattern near the lightflux center of the regions C1 to C4 is not included in other regionsthan the regions of the light receiving parts I and J, thus the lightpattern deviating from the light receiving part 13. In the lightreceiving parts M to P for detecting the focus error signal, as thelight pattern from the region B1 to region B4 and from region C1 toregion C4 become larger, the light pattern deviates from the lightreceiving parts M and P.

A detailed description will be given to how the light pattern changes atthe light receiving parts M to P for detecting the focus error signalwith reference to FIGS. 5A to 5E. FIGS. 5A to 5E show the change of thelight pattern of a light flux a2− which is condensed on the boundarybetween light receiving surfaces O and P during focusing. (b), (c) and(d) correspond to the state shown in FIGS. 4A to 4F, and (a′) shows thestate in the middle between (a) and (b), and (e′) shows the state in themiddle between (d) and (e) in FIG. 4D. At (c), a position correspondingto the light flux center 14 is focused and the light pattern 25 expandsaround the light focus point (light flux center) (c) as the defocusingincreases. At this time, since virtual light patterns 26 and 27corresponding to light fluxes b2− and c2− which are irradiated ontoother light receiving parts expand, the light pattern 25 extends off thelight receiving parts M to P, and the entire light pattern 25 liesoutside the light receiving parts M to P at (a′) and (e′).

In FIGS. 4A to 4F, the interval Wp between the light patterns of theregions A1 to A4 in the X-axis direction during defocusing is determinedin response to the width w between the dividing lines 17 of thepolarizing diffraction grating. Therefore, the relation between theshapes of the light receiving parts 13 and the width w of between thedividing lines 17 is determined such that the light patterns are notirradiated onto the light receiving parts 13 during the defocusing.

Since the light receiving parts 13 are disposed on the interval Wp,light receiving parts 13 can be disposed nearer to each other comparedwith when the light receiving parts 13 are disposed outside the lightpattern during defocusing, thus making it possible to reduce the size ofthe light detector.

Furthermore, if a light pattern of other region is irradiated onto thelight receiving parts M to P when the light patterns expand with theincreasing focusing, a distortion occurs to the focus error signalwaveform, causing an error during focus withdrawal. A shaded area 24 ofthe region A, which is formed by the dividing line 19 which forms anangle of 30 degrees with respect to the Y-axis direction of thepolarizing diffraction grading 5, is provided to prevent the lightpatterns that are irradiated onto the light receiving parts A to D fromentering the light receiving parts M to P when they expand withdefocusing. The shaded area becomes unnecessary depending on thedeposition of the light receiving parts.

FIGS. 6A to 6E show the shapes of light patterns on the light detector12 when the is comprises of two layers, and the waveform of a focuserror signal (FES). A light pattern 28 reflected from the first layer 9is shown by a lattice pattern, while the light pattern reflected fromthe second layer 10 is shown with oblique lines. FIG. 6B corresponds to(a) of FIG. 6A. FIGS. 6C and 6D correspond to (b) of FIG. 6A. FIG. 6Ecorresponds to (c) of FIG. 6A.

The focus error signal waveform (FE) of the two-layer disc is obtainedby combining a focus error signal waveform (FE1) generated at the firstlayer 9 and a focus error signal waveform (FE2) generated at the secondlayer 10. FIG. 6B shows the shapes of light patterns 28 and 29 when thefirst layer 9 is in focus, wherein the light pattern 28 is focused onthe light detector 12, and the light pattern 29 (stray light) isirradiated onto the outside of the light receiving parts 13 at thattime.

As the focus shifts from the first layer 9 to the second layer 10, thesize of the light pattern 28 increases, while the size of the lightpattern 29 diminishes. At the midpoint (b) between the first layer 9 andsecond layer 10, the size of the light pattern 28 and that of lightpattern 29 are substantially the same as Figs. C and D show, and most ofthem are not irradiated onto the light receiving parts M to P. At (c)where the second layer 10 is in focus, the light pattern 29 is focusedonto the light detector 12, and the light pattern 28 (stray light) isirradiated onto the outside of the light receiving parts 13.

While the focus error signal is obtained by computing the outputs of thelight receiving parts M to P, the stray light is not irradiated on thelight receiving parts M to P when the second layer 10 is in focus.Therefore, an offset due to the stray light does not occur there. Inaddition, the offset due to the stray light does not occur even if theintensity distribution of a laser light 2 varies and the light patternis displaced from the light receiving parts 13, thus making it possibleto obtain a stable focus error signal.

Furthermore, if the waveform (FE1) generated at the first layer 9overlaps a large part of the waveform (FE2) generated at the secondlayer 10, a distortion occurs to the focus error signal waveform (FE) ofthe second disc, and sometimes a focus withdrawal error can occur.However, in the embodiment, since the light pattern is hardly irradiatedonto the light receiving parts M to P at a location near the midpointbetween the first layer 9 and the second layer 10, the outputs of the(FE1) and (FE2) are small and thereby distortion experienced by thefocus error signal waveform (FE) is also small.

By the same token, while a tracking error signal is obtained bycomputing the outputs of the light receiving parts A to H and Q to T,the stray light is not irradiated onto the light receiving parts A to Hand Q to T when the layer is in focus, thus making it possible to obtaina stable focus error signal in which an offset due to the stray lightdoes not occur.

Since the light receiving parts I and J include the light flux center, alight near the light flux center remains at the light receiving partsand becomes a stray light even if the light patterns expand. However,this portion is not used for detecting the focus error signal and atracking error signal, and is used only for detecting a reproducingsignal. Therefore, the existence of the stray light causes no problem inpractical use.

Since there is no influence from the stray light as described above, itis possible to change the balance of light amount of plus/minusfirst-order diffracted light which is diffracted at the polarizingdiffraction grating 5. It is possible to improve the SN of a reproducedsignal by increasing the light amount of the plus first-order diffractedlight 22 such that the light amount of the light receiving parts fordetecting the reproduced signal increases. At this time, while the minusfirst-order diffracted light 23 decreases, the offset due to the straylight does not increase because of the reduction in the light amount.Therefore, only electrical restriction has to be considered.

In the present embodiment, the polarizing diffraction grating 5 andone-quarter wave plate 6 may be fixed in one piece with the objectivelens 7 such that they operate together with the objective lens 7.Alternatively, they may separately be fixed so that they do not operatetogether with the objective lens 7. In the case where the polarizingdiffraction grating 5 and one-quarter wave plate 6 are fixed separatelyfrom the objective lens 7, when the objective lens 7 moves in the Xdirection due to the tracking operation, the outside shape of the lightflux, which is shown by an alternate long and two short dashes line inFIG. 2, also moves in the X direction, and the dividing line 15 lies offthe light flux center 14. However, the position and size of the lightpattern interval Wp that appears on the light detector 12 in response tothe width w between the dividing lines 17 do not change. Therefore, thestray light is not irradiated onto the light receiving parts 13 eitherin this case. Since the outside shape of the light flux moves in the Xdirection, the value of K in the computing equation of the trackingerror signal (TES) according to the push pull method differs from thatwhen they are fixed in one piece with the objective lens 7.

The polarizing diffraction grating 5 is not limited to the shape shownin the above embodiment. Other embodiments of the polarizing diffractiongrating will be described below.

Embodiment 2

FIGS. 7 and 8 show the shapes of divided regions of the polarizingdiffraction grating 5 in an embodiment 2 and the shapes of lightpatterns irradiated onto the light detector 12 at that time. The lightpattern 28 of the reflected light from the first layer 9 is focused onthe light detector 12, and the light pattern 29 (stray light) of thereflected light from the second layer 10 at that time is shown byoblique lines. A difference from the embodiment 1 is that the width Wcof the regions C1 to C4 in the X-axis direction is narrower. The regionsB1 to B4 are larger by just that much. The shapes of the regions A1 toA4 are the same as those of the embodiment 1. Since the regions B1 to B4are larger, the stray light is more likely to enter the light receivingparts E to F and Q to T when the light detector 12 is displaced.However, an effect is expected that increases the output of the trackingerror signal.

Embodiment 3

FIGS. 9 and 10 show the shapes of divided regions of the polarizingdiffraction grating 5 in an embodiment 3 and the shapes of lightpatterns irradiated onto the light detector 12 at that time. The lightpattern 28 of the reflected light from the first layer 9 is focused onthe light detector 12, and the light pattern 29 (stray light) of thereflected light from the second layer 10 at that time is shown byoblique lines.

A difference from the embodiment 1 is that there are not the fourdividing lines 18 in the X-axis direction that do not pass through thelight flux center 14, and the four dividing lines 19 that form an angleof 30 degrees with respect to the Y-axis direction extend longer aroundthe light flux center 14. Therefore, the areas of the region B1 to B4are reduced, and thereby the outputs of the light receiving parts Q to Tare reduced. Accordingly, it is necessary to increase the value of K inthe following computing equation for the tracking error signal accordingto the push pull method.

(TES)=((a+e+b+f)−(c+g+d+h))−K((q+r)−(s+t))

However, since the areas of regions A1 to A4 increase, an effect isexpected that increases the output of the error focus signal.

In above embodiment, the polarizing diffraction grating, as a light fluxdividing element, is disposed between the collimate lens and one-quarterwave plate. However, an ordinary diffraction grating may be disposedbetween a polarizing beam splitter and the light detector.

Application can be expected for the optical disc apparatus that recordsand reproduces information on and from an optical disc.

When a target layer of the optical disc is in focus, the stray lightfrom other layer deviates from the light receiving parts for a servosignal of the light detector. Therefore, it is possible to receive onlyreflected light from the target layer to obtain the servo signal, thusmaking it possible to obtain a stable focus error signal and a trackingerror signal free of the offset due to the stray light.

Next, the optical disc apparatus equipped with the optical pickupapparatus according to the present invention will be described.

FIG. 11 shows a schematic diagram of a specific example of the opticaldisc apparatus equipped with the optical pickup apparatus shown inFIG. 1. A semiconductor laser 1, a polarizing beam splitter 3, apolarizing diffraction grating 5, a one-quarter wave plate 6 and a lightdetector 12 corresponding to those shown in FIG. 1, and a mirror 30 forchanging the direction of the laser light, which is not shown in FIG. 1,are bonded and fixed to a case 31. A collimate lens 4 is fixed to thecase 31 such that it can be moved along the optical axis by a motionmechanism 32. The collimate lens 4 can move to a position where thespherical aberration of a laser light 2, which is focused on a lightdisc 8, becomes minimal in each of the cases where recording andreproducing are preformed on a first layer 9 and on a second layer 10 ofthe optical disc 8.

An objective lens 7 is attached to a holder 34 in which a coil 33 isincorporated, and is combined with a magnet, which is not shown, to forman actuator. The objective lens 7 can follow the side-runout anddecentering of the optical disc 8.

The case 31 can be moved in the radial direction of the optical disc 8by a motor 35 and a lead screw 36. The optical disc 8 is fixed to aspindle motor 37.

The operation of each component is controlled by a system controlcircuit 47. When recording or reproducing is performed, the spindlemotor 37 is first driven by the operation of a spindle motor drivingcircuit 46, and then the optical disc 8 is rotated.

Next, the semiconductor laser 1 is radiated by the operation of a laserdriving circuit 41.

Focusing control is performed such that a servo signal generatingcircuit 43 generates a focus error signal from the output of the lightdetector 12, an actuator circuit 45 drives the actuator based on thefocus error signal, and the objective lens 7 focuses the laser light onthe recording and reproducing layer.

When locating the focus point of the laser light 2 on the first layer 9,the focus error signal is detected after the collimate lens 5 is movedto a position corresponding to the first layer 9. Waveforms shown inFIGS. 6A to 6E are obtained to the focus error signal. Therefore,focusing control is performed such that the focus point is located onthe first layer.

Next, an access control circuit 44 is operated to rotate the motor 35,and the case 31 is moved to a desired position on the inner periphery orouter periphery of the optical disc through the lead screw 36.

Then, tracking control is performed in which the actuator circuit 45drives the actuator based on the tracking error signal generated by theservo signal generating circuit 43 from the output of the light detector12 to follow the focus point of the laser light 2 on the track of theoptical disc 8.

Then, data on the track of the optical disc 8 is reproduced from theoutput of the light detector 12 by an information signal generatingcircuit 42.

When information is recorded on the optical disc 8, a laser drivingcircuit 41 is operated by the system control circuit 47 in response tothe information to be recorded, and a record mark is formed on the trackby modulating the output of the semiconductor laser 1.

When moving the recording and reproducing layer from the first layer 9to the second layer 10, the focus point of the laser light 2 is movedtowards the second layer by stopping the focusing control and operatingthe actuator driving circuit 45 at the same time after the trackingcontrol 45 is stopped by the system control circuit 47. Then, thefocusing control is performed such that the actuator is driven at thetiming that the focus point position of the second layer of the focuserror signal is detected and the focus point of the laser light islocated on the second layer. Then, the tracking control is performed inwhich after the collimate lens 4 is moved to a position corresponding tothe second layer 10, the actuator is driven based on the tracking errorsignal to follow the focus point of the laser light 2 on the track. Thereproducing operation and recording operation are performed on thesecond layer 10 in the same way as on the first layer 9.

While the polarizing diffraction grating 5 and one-quarter wave plate 6are fixed to the case 31 in the above embodiment, they may be fixed tothe holder 34, to which the objective lens 7 is fixed, such that theymove together with the objective lens 7.

While the optical pickup apparatus and optical disc apparatus equippedwith the same according to the present invention have been described indetail by way of embodiments thereof in the above, the present inventionis not limited to the above embodiments. The present invention caninclude various variations and improvements without departing from thespirit of the present invention.

For example, while recording or reproducing on or from the optical discin which two layers of recording and reproducing layer (informationrecording layer) are laminated in the above embodiments, the presentinvention is also adaptable to recording or reproducing on or from anoptical disc in which three layers or more of recording and reproducinglayer are laminated.

Furthermore, the disposition pattern of light receiving parts of thelight detector is not limited to the above examples. The light receivingparts may be disposed in any way unless a reflected light flux fromother recording and reproducing layer than the target recording andreproducing layer is not irradiated onto the light receiving parts ofthe light detector when the target information recording layer of theoptical disc is in focus.

In addition, while the first divided region comprises four regions of C1to C4 in the above embodiments, the present invention is not limited tothe same. The first divided region may comprise only one region, tworegions, or four or more regions.

Next, embodiments of the optical pickup apparatus according to thepresent invention will be described.

Embodiment 4

FIG. 12 is a schematic diagram showing an exemplary optical pickupapparatus according to the present invention.

The optical pickup apparatus 101 is structured such that it can bedriven by a drive mechanism 107 in the radial direction of the opticaldisc 100 as is shown in FIG. 12. An actuator 105 on the optical disc 100is equipped with an objective lens 102. The optical disc 100 isirradiated with light by the objective lens 102. The light emitted fromthe objective lens 102 forms a spot on the disc and is reflected fromthe disc. A focus error signal and a tracking error signal are generatedby detecting the reflected light.

FIG. 13 shows an optical system for the above optical pickup apparatus.While BD will be described here, HD-DVD and other recording methods arealso adaptable.

A light flux with a wavelength of about 405 nm is emitted from asemiconductor laser 50 as a divergent light. The light flux emitted fromthe laser 50 is converted by a collimate lens 51 into a substantiallyparallel light. The light flux that passes through the collimate lens 51is reflected by a beam splitter 52. Part of the light flux passesthrough the beam splitter 52 to enter a front monitor 53. Generally,when information is recorded on a recording type optical disc such asRD-RE or BD-R, a given amount of light is irradiated onto the recordingsurface of the optical disc with. Therefore, it is necessary to highlyprecisely control the light amount of the semiconductor laser. For thepurpose, the front monitor 53 detects a change in the light amount ofthe semiconductor laser 50 when information is recorded on the recordingtype optical disc, and feeds back the result to the a drive circuit (notshown) of the semiconductor laser 50. This enables monitoring the lightamount on the optical disc.

The light flux reflected from the beam splitter 52 enters a beamexpander 54. The beam expander 54 has a function to change the divergingor converging state of the light flux. Therefore, the beam expander 54is used for compensating the spherical aberration due to an error inthickness of a cover layer of the optical disc 100. The light fluxemitted from the beam expander 54 is reflected by a start-up mirror 55and passes through a one-quarter wave plate 56, and thereafter the lightflux is focused on the optical disc 100 by the objective lens 102mounted on the actuator 105.

The light flux reflected by the optical disc 100 passes through theobjective lens 102, one-quarter wave plate 56, start-up mirror 55, beamexpander 54 and beam splitter 52. The light flux passing through thebeam splitter 52 is separated into a light flux passing through a beamsplitter 57 and a light flux reflected by the beam splitter 57.

A focus error signal is detected from the light flux reflected by thebeam splitter 57 according to a knife edge method. It should be notedthat the knife edge method is used here as a focus detecting method, butnot limited to the knife edge method. Since the knife edge method ispublicly known, its description is omitted here. After passing throughthe beam splitter 57, the light flux enters a light detector 108. Thelight detector 108 detects a signal on the disc and a tracking errorsignal.

FIG. 14 shows a pattern of a light receiving part 108 according to thepresent invention. The light receiving part 108 comprises four regionsof a region I (region 1), a region J (region 2), a region G (region 3)and a region H (region 4). The region I (region 1) and region G (region3) are line-symmetrical to the region J (region 2) and region H (region4) with respect to the center line of the light receiving part. Inaddition, the region I (region 1) and region J (region 2) arecharacterized in that their widths in the central axis 500 directionbecome narrower with the distance away in the direction substantiallyperpendicular to the central axis 500 from the central axis (or centerline) 500.

Here, the principle of detecting a tracking error signal of the one beammethod will be described with reference to FIG. 14. On the detectorsurface, there appears a region where a 0th-order diffracted light and aplus/minus first-order diffracted light interfere with each other. Theinterfering state of the regions differs depending on the spot positionson the track. Therefore, this can be used to dispose a spot on a desiredtrack position. Actually, a push pull signal is generated by computingthe difference between a signal obtained at an interference region Z1 ofthe 0th-order diffracted light and plus first-order diffracted light,and a signal obtained at an interference region Z2 of the 0th-orderdiffracted light and minus first-order diffracted light. The signal inthe regions other than the interference hardly depends upon thepositions of the spot on the track. The one beam method uses thischaracteristic.

The foregoing will be described in detail hereinafter. The light flux isdisplaced on the light receiving part in the arrow direction of FIG. 14with the displacement of the objective lens, and intensity distributionis also displaced in the same direction at the same time. A DC offsetoccurs to the signal of (I-J) due to the two effects. The DC offset alsooccurs to the signal of (G-H). FIG. 15 shows the amount of offset of the(I-J) signal and (G-H) signal relative to the amount of displacement ofthe objective lens. It is known from FIG. 15 that the amount of DCoffset occurring to the (I-J) signal and (G-H) signal relative to thedisplacement of the objective lens is nearly linear. Therefore, it isknown that a tracking error signal in which DC offset is suppressed canbe detected by performing the following computation.

(Tracking error signal)=(I−J)−k·(G−H)  (equation 1)

where k is a coefficient for correcting the DC offset of the (I−J)signal and DC offset of the (G−H) signal. In this manner, the one beammethod enables the detection of the tracking error signal in whichoffset is suppressed.

Next, description will be made to the offset of a tracking error signaloccurring at the boundary between an unrecorded region and a recordedregion on the disc. FIGS. 16A to 16C schematically show the waveform ofa tracking error signal according to the one beam method at the boundarybetween the unrecorded region and recorded region. FIG. 16A shows whenthe offset can not be suppressed. FIG. 16B shows when the offset occurson the reverse side due to overcorrection. FIG. 16C shows a trackingerror signal in which the offset is suppressed.

In the waveforms of the tracking error signals shown in FIGS. 16A and16B, the tracking error signals are more likely not to cross theoriginal point position due to variations or the like. If the trackingerror signal does not cross the original point position, it is a problemin terms of servo control. (Tracking control is performed by performingthe servo control at the original point position). Therefore, it isevident that the waveform shown in FIG. 16C is desirable.

Here, the bottom ratio and top ratio are considered as an indicator forthe offset in the tracking error signal. The bottom ratio is assumed tobe (a−c)/(c+d) as shown in FIGS. 16A to 16C. This indicates that, thedifference is first obtained between a tracking error signal bottomvalue in the recording region and a tracking error signal bottom valueat the boundary between the unrecorded region and recorded region, andthen the difference is divided by the tracking error signal amplitude inthe recorded region. Specifically, it indicates how much bottom lies inthe lower side when compared with the tracking error signal amplitude inthe recording region. In contrast, the top ratio is assumed to be(b−d)/(c+d). This indicates that the difference is first obtainedbetween a tracking error signal top value in the recording region and atracking error signal top value at the boundary between the unrecordedregion and recorded region, and then the difference is divided by thetracking error signal amplitude in the recorded region. Specifically, itindicates how much top lies in the upper side when compared with thetracking error signal amplitude in the recording region.

If the two indicators are positive values, the tracking error signalamplitude gradually changes even if a spot shifts from the unrecordedregion to the recorded region, and thereby the servo control stabilizesas is known from FIG. 16C. However, if one indicator is a positive valueand the other one is a negative value, the offset can not suppressed,posing a problem. The DC offset also occurs to the tracking error signalwhen the objective lens is displaced in the tracking direction.Accordingly, the offset due to the boundary between the unrecordedregion and recorded region and to the displacement of the objective lensmust be suppressed simultaneously.

The evaluation of the offset of the tracking error signal that occurs atthe boundary between the unrecorded region and recorded region when theobjective lens is displaced will be performed based on the aboveindicators in the following sections. Here, the calculation conditionswhen performing simulation are as follows.

wavelength: 405 nmobjective lens NA: 0.85track pitch: 0.32 μmobjective lens focal length: 1.41 mm

FIG. 17A shows the ratio of top when the objective lens is displaced inthe tracking direction while the light receiving parts of the presentinvention and JP-A-9-223321 are used. FIG. 17B shows the ratio of bottomin the same situation. The conditions of the light receiving parts ofthe present invention are as follows. t1=d2/d1=0.19, where t1 is theratio of the interval between the region I (region 1) and region J(region 2) relative to the diameter of the light flux incident to thedetector. t2=d3/d1=0.5, where t2 is the ratio of the maximum width ofthe region I (region 1) and region J (region 2) in the center axisdirection relative to the diameter of the light flux incident to thedetector. The slope angle θ of the outside shape of the region I (region1) and region J (region 2) with respect to the direction perpendicularto the center axis is assumed to be 10 degrees.

FIGS. 17A and 17B show that the top ratio assumes negative values atmost of the objective lens displacement amount in the case ofJP-A-9-223321. In addition, FIGS. 17A and 17B show that the bottom ratioassumes positive values. This indicates that the offset is occurring.

In contrast, in the case of the present invention, both the top ratioand bottom ratio are positive in most of the objective lens displacementamount, indicating that the offset at the boundary between the recordedregion and unrecorded region is suppressed.

Next, effects will be described that are provided by inclining thedividing lines of the region I and J. FIGS. 18A and 18B show the resultof the simulation performed when the dividing lines are inclined. FIG.18A shows the top ratio, while FIG. 18B shows the bottom ratio. Theconditions of the light receiving parts are t1=0.19, t2=0.5 and θ=10degree, and t1=0.19, t2=0.5, and θ=0 degree.

The top ratio assumes positive values in most regions where theobjective lens is displaced, and the bottom ratio is significantlyimproved at the regions where the objective lens displacement isnegative. In this manner, inclined dividing lines would be able tosuppress the offset at the boundary between the unrecorded region andrecorded region. Especially, a larger improvement effect will beprovided in suppressing the DC offset and the offset at the boundarybetween the unrecorded region and recorded region when 0 degree<θ<15degree, and 0<t1<0.35, and 0<t2<0.70, where t1 and t2 are figuresrelative to the diameter of the light flux entering the light receivingparts of the light detector 10.

When simply thinking, as shown in FIG. 14, the tracking error signalamplitude seems to extremely decrease if the widths of the regions I andJ in the direction of the center axis 500 become smaller as the regionsI and J move away from the center axis 500 in the substantiallyperpendicular direction, because when the objective lens is displaced,the area to be detected decreases in the interference regions(interference region Z1 or interference region Z2) on the lightreceiving parts region (region I or region J) in the direction of theobjective lens displacement. However, actually, when the objective lensis displaced, the intensity distribution of the light flux is displacedat the same time in the objective lens displacement direction by twotimes the objective lens displacement amount. Therefore, while the areadecreases, the intensity increases on the light receiving part region(region I or region J) in the objective lens displacement direction.Moreover, while the intensity decreases, the area increases on the lightreceiving part region (region J or region I) opposite to the objectivelens displacement direction. Therefore, the tracking error signalamplitude is less prone to decrease, and the DC offset is more likely tobe corrected. Moreover, the offset at the boundary between theunrecorded region and recorded region greatly occurs at a location nearthe interference region (interference region Z1 or interference regionZ2). Accordingly, it is effective in suppressing the offset at theboundary between the unrecorded region and recorded region to enter thelight of the regions other than the interference region onto the lightreceiving parts on the DC offset detection side when the objective lensis displaced.

While FIG. 14 shows the dividing lines inside the detector by straightlines that are substantially parallel with the track, and straight linesthat extend from there to form other angles, it does not matter at allwhether the dividing lines inside the detector are arc lines as shown inFIG. 19A, or straight lines as shown in FIG. 19B. Furthermore, while thepattern of the light receiving parts is shown here, it is needless tosay that a similar effect is provided by disposing a diffraction gratinghaving the same pattern as that of the light receiving parts like theoptical system of FIG. 20, and by changing the diffraction directionsand angles in each region to detect signals with a plurality of lightreceiving parts.

Embodiment 5

FIG. 21 shows a pattern of the light receiving part which relates to anembodiment 5 and differs from that of the embodiment 4. A differencefrom the embodiment 4 lies in that the pattern of the embodiment 5 isprovided with a center region Y (region 5). The ratio of the length inthe center axis direction of the center region relative to the diameterof the light flux entering the light receiving part of the detector 108is t3. The ratio of the length in the direction perpendicular to thecenter axis of the center region relative to the diameter of the lightflux entering the light receiving part of the detector 108 is t4. Thelight receiving part is capable of generating a tracking error signal byperforming the following computation.

(tracking error signal)=(C−D)−k·{(A−B)+(E−F)}  (equation 2)

FIG. 22A and FIG. 22B show the top ratio and bottom ratio, respectively,when the light receiving parts of the present invention andJP-A-9-223321 are used, and the objective lens is displaced in thetracking direction. The light receiving parts of the present inventionare calculated under the condition that t1=0.19, t2=0.54, t3=0.19,t4=0.19 and θ=10 degree.

As FIGS. 22A and 22B show, the tracking error signal of JP-A-9-223321changes a lot with a change in the characteristic at the boundarybetween the unrecorded region and recorded region and the displacementof the objective lens. In contrast, the tracking error signal of thepresent invention does not change with the displacement of the objectivelens, thus making it unnecessary to control particularly thedisplacement of the lens. Both the top ratio and bottom ratio arepositive in most of the objective lens displacement amount, indicatingthat the offset at the boundary between the recorded region andunrecorded region is suppressed.

The use of the detector pattern such as that of the present inventionenables stable tracking control even if the objective lens is displaced.The DC offset as well as the offset at the boundary between theunrecorded region and recorded region are particularly effectivelysuppressed under the condition that 0 degrees<θ<15 degrees, 0<t1<0.35,0<t2<0.70, 0<t3<0.35 and 0<t4<0.35, where t1, t2, t3 and t4 are ratiosrelative to the diameter of light flux entering the light receivingparts of the detector 10.

As described in the embodiment 4, the offset of the boundary between theunrecorded region and recorded region occurs greatly at locations nearthe interference region (interference region Z1 or interference regionZ2), and the center part of the detecting surface occurs little offset.Furthermore, since the region is not detected for a tracking errorsignal, the coefficient k can be set to an appropriate value. As aresult, it is possible to improve the effect of suppressing the offsetat the boundary between the unrecorded region and recorded region.

While the dividing lines inside the detector are shown in straight linesthat are substantially parallel with the track and straight lines thatextend from there to form angles in FIG. 21, the dividing lines insidethe light receiving part could be arc lines as shown in FIG. 23A, orstraight lines as shown in FIG. 23B.

While patterns of the light receiving parts are shown here, it isneedless to say that similar effects are provided by disposing adiffraction grating 61 having the same pattern as that of the lightreceiving parts shown in FIG. 4 to detect signals with a plurality oflight receiving parts on the detector.

Embodiment 6

FIG. 24 shows an optical system of an optical pickup apparatus relatingto an embodiment 6 of the present invention. In the embodiment 6, likenumerals are used for like and corresponding parts of the embodiment 4of the present invention shown in FIG. 13. Although BD will be describedhere, it could be HD-DVD or other recording type methods.

A light flux with a wavelength of about 405 nm is emitted from asemiconductor laser 50 as a divergent light. The light flux emitted fromthe laser 50 is reflected by a beam splitter 52. Part of the light fluxpasses through the beam splitter 52 to enter a front monitor 53. Thelight flux reflected by the beam splitter 52 is converted by a collimatelens 51 into a substantially parallel light flux. The light flux passingthrough the collimate lens 51 enters a beam expander 54. The light fluxemitted from the beam expander 54 is reflected by a start-up mirror 55,passes through a one-quarter wave plate 56 and is condensed on anoptical disc 100 by an objective lens 102 mounted on an actuator 105.

The light flux reflected by the optical disc 100 passes through theobjective lens 2, one-quarter wave plate 56, start-up mirror 55, beamexpander 54, collimate lens 51 and beam splitter 52.

The light flux passing through the beam splitter 52 is divided by adiffraction grating 63 into a light flux for generating a focus errorsignal (0th-order diffracted light) and a light flux for generating atracking error signal (plus first-order diffracted light or minusfirst-order diffracted light). While a description is made here usingthe diffracting grating of FIG. 14 relating to the embodiment 4, thediffraction grating of FIG. 19A or FIG. 19B relating to the embodiment4, or that of FIG. 21, FIG. 23A or FIG. 23B relating to the embodiment 5can also be used. The light flux divided by the diffraction grating 63enters a detection lens. The light flux divided by the diffractiongrating 63 enters a detection lens. When passing through the detectionlens, the light flux is given a predetermined astigmatism, which is usedfor the detection of the focus error signal. The light flux forgenerating the tracking error signal is given an astigmatism andspherical aberration when diffracting the diffraction grating 63.Therefore, the light flux passing through the detection lens 59 iscondensed on the light receiving parts.

FIGS. 25A and 25B show a detector 64 and light fluxes to be detected.The detector 64 is divided into focus detecting regions 40 to 43 andtracking error signal regions 44 to 47. Since the focus error signalsare publicly known, its description is omitted here. The directions ofthe light fluxes that diffracted the diffracting grating are differentin each region, and a light flux diffracting a region G of FIG. 14enters a region 45 of FIG. 25B, a light flux diffracting a region Henters a region 46, a light flux diffracting a region I enters a region44, and a light flux diffracting a region J enters a region 47. Thiscauses the tracking error signals to be generated. Here, the RF signalcan be detected by obtaining the total of the focus error signals, thetotal of the tracking error signals, or the total of the focus errorsignals and tracking error signals. The use of the regions 40 to 43 forfocus error signals would also enable DPD (Differential Phase Detection)based on the tracking error signal detection method which is adopted fora DVD-ROM or the like.

With such an optical system structure as described above, it becomespossible to obtain not only the tracking error signals but also othersignals. While the diffracting grating 63 is disposed on the detectorside here, instead a polarizing diffraction grating 65 can be disposednear the objective lens as shown in FIG. 26.

Embodiment 7

FIG. 27 shows an optical system of an optical pickup apparatus relatingto an embodiment 7 of the present invention. In the embodiment 7, likenumerals are used for like and corresponding parts of the embodiment 4shown in FIG. 13. Although BD will be described here, it could be HD-DVDor other recording type methods instead.

A P-polarization light flux with a wavelength of about 405 nm is emittedfrom a semiconductor laser 90 as a divergent light. The light fluxemitted from the laser 90 passes through the beam splitter 91 and isreflected by a mirror 92. Part of the light flux outside the pitchdiameter enters a front monitor 93. The light flux reflected by themirror 92 enters an auxiliary lens 94 and then a collimate lens 95. Thecollimate lens 95, which can be driven in the light axis direction by adriving mechanism (not shown), can change the diverging or convergingstate of the light flux thereby to compensate the spherical aberrationdue to the thickness error of a covering layer of an optical disc 100.

The P-polarization light flux passing through the collimate lens 95enters a polarizing diffraction grating 66 of the present invention. TheP-polarization light flux that entered the polarizing diffractiongrating 66 passes through the diffraction grating 66, is reflected by astart-up mirror 96, passes through a one-quarter wave plate 97, andthereafter becomes a circularly polarized light. The light flux thatbecame a circularly polarized light is condensed on the optical disc 100by the objective lens 102 which is equipped with an actuator 105.

The light flux reflected by the optical disc 100 passes through theobjective lens 102 and one-quarter wave plate 97. The circularlypolarized light is converted into an S-polarized light by theone-quarter wave plate 97. The S-polarized light flux is reflected bythe start-up mirror 96 and enters the polarizing diffraction grating 66.The S-polarized light entering the polarizing diffraction grating 66 isdivided by the polarizing diffraction grating 66 into a plurality oflight fluxes. The light fluxes passing through the polarizingdiffraction grating 66 are reflected by the beam splitter after passingthrough the collimate lens 95, auxiliary lens 94 and mirror lens 92, andthen enters a detector 67.

FIGS. 28 and 29 show patterns of the polarizing diffraction gratingwhich consider the tracking error signals as well as focus errorsignals. FIGS. 27 and 29 show detector 67. In the polarizing diffractiongrating 66 is a diffracting grating in which only plus/minus first-orderlight is diffracted, and the diffraction direction and diffraction angleof each of the diffracted gratings are different in each region. For thesake of simplicity, FIG. 29 shows the light fluxes that are diffractedfrom each region of the polarizing diffraction grating shown in FIG. 28by means of characters of the regions. Additionally, a subscript “+”added to the character indicates a plus first-order diffracted light,while a subscript “−” added to the character indicates a minusfirst-order diffracted light. For example, a plus first-order diffractedlight of a region L of the polarizing diffraction grating 66 of FIG. 28enters a region 74 of a detector 67 of FIG. 29, and a minus first-orderdiffracted light enters a region 83.

The focus error detection method is based on the knife edge method.Detection is performed by the minus first-order diffracted lightdiffracted in regions N, P, Q and O of the polarizing diffractiongrating 66. Since the knife edge method is publicly known, itsdescription is omitted here. The detection of the tracking error signalcan be obtained by performing the following computation using thedetection signals of regions 70 to 79 and regions 81 to 84.

(Tracking error signal)={(N ₊ +L ₊)+(P ₊ +R ₊)−(O ₊ +M ₊)+(Q ₊ +S₊)}−k·{(L ⁻ +R ⁻)+(M ⁻ +S ⁻)}  (equation 3)

While the polarizing diffraction grating 66 is divided into a pluralityof regions for the purpose of detecting focuses or the like, it is thesame detection method as FIG. 21 of the embodiment 2 from the viewpointof the tracking error signal. Moreover, the RF signal detection isobtained by performing the following computation using the detectionsignals of regions 70 to 79.

(RF signal)=N ₊ +P ₊ +Q ₊ +O ₊ +L ₊ +R ₊ +S ₊ +M ₊ +T ₊ +U ₊  (equation4)

DPD signal detection is also obtained by performing the followingcomputation using the detection signals of regions 70 to 79.

(DPD signal)={(N ₊ +L ₊)+(Q ₊ +S ₊)}−{(P ₊ +R ₊)+(O ₊ +M ₊)}  (equation5)

Such an optical system structure enables obtaining not only the trackingerror signals but also other signals. Embodiment 8

In an embodiment 8, an optical reproducing apparatus equipped with anoptical pickup apparatus 101 will be described. FIG. 30 is a schematicstructure of the optical reproducing apparatus. The optical pickupapparatus 101 is provided with a mechanism for allowing the opticalpickup apparatus to move in the radial direction of the optical disc andis position-controlled in response to an access control signal from anaccess control circuit 172.

A predetermined laser driving current is supplied to a semiconductorlaser in the pickup apparatus 101 from a laser lighting circuit 177, anda laser light of a predetermined light amount is emitted from thesemiconductor laser in response to reproduction. It should be noted thatthe laser lighting circuit 177 can be installed in the optical pickupapparatus 101.

A signal outputted from a light detector in the optical pickup apparatus101 is transferred to a servo signal generating circuit 174 andinformation signal generating circuit 175. A servo signal such as afocus error signal, a tracking error signal or a tilt control signal isgenerated at the servo signal generating circuit 174 based on the signalfrom the light detector. An objective lens is position-controlled bycontrolling an actuator in the pickup apparatus 101 via the actuatorcircuit 173 based on the servo signal.

At the information signal reproducing circuit 175, information signalsstored in the optical disc 100 are reproduced based on the informationfrom the light detector. Part of the signals obtained at the servosignal generating circuit 174 and information reproducing circuit 175 istransferred to a control circuit 176. A spindle motor driving circuit171, the access control circuit 172, the servo signal generating circuit174, the laser lighting circuit 177, a spherical aberration correctionelement driving circuit 179 and the like are connected to the controlcircuit 176. The control circuit 176 controls the rotation, accessdirection and access position of a spindle motor 180 that rotates theoptical disc 100, servo-controls the objective lens, controls the amountof light emitted by the semiconductor laser in the optical pickupapparatus 101, corrects the spherical aberration due to a difference inthe disc thickness, and performs others.

Embodiment 9

In an embodiment 9, an optical recording and reproducing apparatusequipped with an optical pickup apparatus 101 will be described. FIG. 31is a schematic structure of the optical recording and reproducingapparatus. A difference of the optical recording and reproducingapparatus of the embodiment 9 from the optical information reproducingapparatus shown in FIG. 30 lies in that the apparatus of this embodimentis provided with an information signal recording circuit 178 between thecontrol circuit 176 and laser lighting circuit 177, and is added with afunction for controlling the lighting of the laser light circuit 177based on the record controlling signal from the information signalrecording circuit 178 to write desired information to the optical disc100.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An optical pickup apparatus, comprising: a semiconductor laser foremitting a laser light; an objective lens for irradiating the light fluxemitted from said semiconductor laser onto an optical disc; and a lightdetector for receiving the light flux reflected from the optical disc,wherein said light detector has a light receiving part that comprisesfour regions of a region 1, a region 2, a region 3 and a region 4; theregion 1 and region 3 are line-symmetrical to the region 2 and region 4with respect to the center line of said light receiving part; the widthof the region 1 and region 2 of said light receiving part becomesnarrower with the distance away from said center line; and signalsdetected in the region 1, region 2, region 3 and region 4 of said lightreceiving part are outputted.
 2. The optical pickup apparatus accordingto claim 1, wherein the outside shape of said region 1 and region 2 hasa side having a slope angle of θ with respect to a directionperpendicular to the center line, and the θ satisfies 0 degrees <θ<15degrees.
 3. An optical pickup apparatus, comprising: a semiconductorlaser for emitting a laser light; an objective lens for irradiating thelight flux emitted from said semiconductor laser onto an optical disc; adiffraction grating for dividing the light flux reflected from theoptical disc; and a light detector for receiving the light flux dividedby said diffraction grating, wherein said diffraction grating has fourregions of a region 1, a region 2, a region 3 and a region 4; the region1 and region 3 are line-symmetrical to the region 2 and region 4 of saiddiffraction grating with respect to the center line of said diffractiongrating; the region 1 and region 2 of said diffraction grating becomenarrower as the distance away from said center line; and signalsdetected from the diffracted lights of the region 1, region 2, region 3and region 4 of said diffraction grating are outputted.
 4. An opticaldisc apparatus, comprising: an optical pickup apparatus comprising: asemiconductor laser for emitting a laser light; an objective lens forirradiating the light flux emitted from said semiconductor laser onto anoptical disc; and a light detector for receiving the light fluxreflected from the optical disc, wherein, said light detector has alight receiving part that comprises four regions of a region 1, a region2, a region 3 and a region 4; the region 1 and region 3 areline-symmetrical to the region 2 and region 4 with respect to the centerline of said light receiving part; the width of the region 1 and region2 of said light receiving part becomes narrower with the distance awayfrom said center line; and signals detected in the region 1, region 2,region 3 and region 4 of said light receiving part are outputted; and aservo signal generating circuit for generating a tracking error signalusing a signal outputted from said optical pickup apparatus, whereinsaid servo signal generating circuit generates the tracking error signalaccording to(A1−A2)−k×(A3−A4) wherein, A1 is a signal corresponding to a lightentered said region 1 and detected by said detector; A2 is a signalcorresponding to a light entered said region 2 and detected by saiddetector; A3 is a signal corresponding to a light said region 3 anddetected by said detector; A4 is a signal corresponding to a lightentered said region 4 and detected by said detector; and k is acoefficient for correcting amounts of light.
 5. An optical discapparatus, comprising: an optical pickup apparatus comprising: asemiconductor laser for emitting a laser light; an objective lens forirradiating the light flux emitted from said semiconductor laser onto anoptical disc; a diffraction grating for dividing the light fluxreflected from the optical disc; and a light detector for receiving thelight flux divided by said diffraction grating, wherein said diffractiongrating has four regions of a region 1, a region 2, a region 3 and aregion 4; the region 1 and region 3 are line-symmetrical to the region 2and region 4 of said diffraction grating with respect to the center lineof said diffraction grating; the region 1 and region 2 of saiddiffraction grating become narrower as the distance away from saidcenter line; and signals detected from the diffracted lights of theregion 1, region 2, region 3 and region 4 of said diffraction gratingare outputted; and a servo signal generating circuit for generating atracking error signal using a signal outputted from said optical pickupapparatus, wherein said servo signal generating circuit generates thetracking error signal according to(A1−A2)−k×(A3−A4) wherein, A1 is a signal corresponding to a lightentered said region 1 and detected by said detector; A2 is a signalcorresponding to a light entered said region 2 and detected by saiddetector; A3 is a signal corresponding to a light said region 3 anddetected by said detector; A4 is a signal corresponding to a lightentered said region 4 and detected by said detector; and k is acoefficient for correcting amounts of light.